Heretofore, many schemes have been devised for amplifying a pulsed or continuous-wave laser beam. As used herein, a "laser amplifier" is a device operable to increase the energy of a light beam by exploiting the principle of stimulated emission.
Early laser amplifiers were single-pass, meaning that the incident light beam passed only once through an "active medium." An active medium as used in a laser amplifier according to the present invention is a substance that, upon receiving a suitable dose of energy, exhibits stimulated emission of radiation that can boost the energy level of (i.e., "amplify") a beam passing through the substance. An active medium is also termed a "gain medium" or "amplifying medium" because the substance imparts "gain" (i.e., an increase in energy level or an amplification) to the beam passing through the gain medium. In many instances, the amount of gain imparted to an incident beam passing once through an active medium in a laser amplifier is small. Hence, many prior-art laser amplification schemes involve passing the laser beam through multiple laser amplifiers in a cascade arrangement to produce sufficient cumulative gain for the intended use of the laser beam. Unfortunately, such schemes tend to be large and expensive.
Other prior-art schemes involve "multiple-pass" laser amplifiers in which the incident beam is made to pass multiple times through a unit of gain medium. Multiple-pass arrangements can impart substantial cumulative gain to a laser beam without the need for cascading and with relatively high efficiency. The beam is typically routed, using multiple reflective mirrors, back and forth or around one or more circumferential paths having any of various desired geometric profiles. (Each such circumferential path is termed herein a "ring".) In instances in which the beam makes multiple circumferential traversals of the same ring, each traversal around the ring is termed herein a "cycle." Cycles can be configured "off-axis" allowing, for each cycle, the beam to impinge upon and thus pass through the unit of gain medium at a slightly different angle. For representative prior-art multiple-pass laser amplification schemes, reference is made, for example, to U.S. Pat. No. 3,365,671 to Kogelnik, and U.S. Pat. No. 4,156,852 to Hagen.
In conventional multiple-pass laser amplifiers, the beam may exhibit any of various undesirable spatial intensity fluctuations such as ripples, rings, and holes. These fluctations represent a substantial departure from a normally ideal Gaussian transverse intensity profile of the beam, and can arise, for example, from diffraction and scattering by optical defects and particles in the air or on optical surfaces. These fluctuations in multiple-pass amplifiers tend to be cumulative and can result in serious degradation of the beam and damage to optical components of the amplifier.
Additionally, prior-art multiple-pass laser amplifiers suffer from various drawbacks and thus are limited to certain applications. For example, regenerative amplifier configurations have proven to be difficult to use for producing laser pulses having both high energy and extremely short duration. This is due, inter alia, to the relatively broad spectral bandwidth of short pulses. Regenerative amplifier configurations employing Ti:sapphire crystals are disclosed in, e.g., Rudd et al., Opt. Lett. 18:2044 (1993); and Wynne et al., Opt. Lett. 19:895 (1994). Regenerative amplifier configurations tend to have a large number of optical components that collectively operate over spectral bandwidths that simply are too narrow for producing extremely short pulses.
Ti:sapphire laser amplifiers have been developed that use the technique of chirped-pulse amplification to attain high peak powers (e.g., terawatt) with extremely short (e.g., femtosecond) pulse durations. Strickland et al., Opt. Commun. 56:219 (1985); and Zhou et al., Opt. Lett. 20:64 (1995). Such laser systems are capable of producing terawatt power levels, but at repetition rates of 10 Hertz or less. Also, such devices are typically bulky. For certain applications, a higher repetition rate would be advantageous, even with some sacrifice of the peak power.
Thus, there is a need for further refinement of laser amplifiers, including laser amplifiers capable of producing high-energy laser pulses of extremely short (femtosecond) duration and high (kHz) repetition rate, and excellent beam quality.